The invention relates to a clamping device for an axially displaceable load-bearing rod, particularly for use in braking or immobilizing machine parts, preferably displaceable by a hydraulic drive, and of the type comprising one or preferably a plurality of clamping jaws which are displaceable in a housing on respective guide tracks inclined relative to the axis of the rod and which are connected to a servo-drive.
Clamping devices of the above-described type are used for immobilizing or braking machine parts or loads which are arranged to be movable by means of a rod, and particularly by way of a piston-rod. Such devices may be used as setting means for machine-tool parts and also as means for adjusting the level of components of various kinds which are moved from a travel position to fixed supports. A clamping device of this kind may also be used as a safety means for hydraulically lifted piston-rods in hoists, lifting platforms and the like, wherein a sudden unintentional drop in pressure of the hydraulic drive medium may cause a dangerous interruption of operations.
The prior art includes a clamping device wherein the clamping jaws are applied to the drive rod by relaxing a powerful spring. The clamping device is released when the spring force is hydraulically overcome by a pressure-applying piston. With such a clamping device, gripping and release of the drive rod is possible without additional movement of the rod, i.e. particularly without lowering a load. Furthermore the clamping device maintains its hold by spring force even in the event of power failure and drop in pressure. However, because of the relatively low spring force, imposed by design, only a relatively short distance of radial movement of the clamping jaws can be achieved. This calls for hard friction faces and involves the danger of the clamping jaws seizing on the drive rod. The relatively heavy springs increase the size of the construction, and failure of the springs immediately causes total removal of the clamping action.
DE-Ps 23 33 491, corresponding to U.S. Pat. No. 3,995,534, discloses a catch device, forming a safety means for hydraulically lifted piston-rods in lift cabins or on crane platforms, in which device the piston-rod is provided with a brake device, which is actuated by the pressure of the operating medium and is provided with a plurality of brake jaws which are under direct spring bias. The brake jaws, in the form of sectors of a circle, surround the piston-rod and are displaceably mounted in the housing on guide tracks which are inclined relative to the central axis. The brake jaws are moved by way of associated hydraulic drives which are controlled by the operating medium. In this catch device, the use of inclined guide tracks permits the brake jaws to execute great axial and radial movements, so that use can be made of soft brake linings in an arrangement that is not affected by differences in sizes due to manufacturing tolerances, so that no metallic surfaces that rub against each other are present. Because of the clamping forces set up when the load is lowered, a construction of small volume is achieved, and the catch device is self-locking, because of the action of the load, even if the supply of the servo-medium fails.
However, in the case of this known catch device, a suitable movement of the load is necessary both for using an cancelling the clamping action, and for certain purposes this movement cannot be carried out. If, for example, a machine part or apparatus part is first lifted into a specific position and if this position is to be secured by clamping, no additional movement of the load should take place for the purpose of effecting the clamping action.